Electron beam devices, in particular scanning electron microscopes are used, for example to examine surfaces of objects (specimens). In a scanning electron microscope, a primary electron beam having electrons is to this end generated using the beam generator. The primary electron beam is focused onto the object to be examined by an objective lens. It is also known to guide the primary electron beam in a raster-scan pattern over the surface of the object to be examined using a deflection device. In the process, the electrons of the primary electron beam interact with the object. As a result of the interaction, in particular electrons are emitted from the object (what is referred to as secondary electrons), or electrons of the primary electron beam are scattered back (what is referred to as back-scattered electrons). The back-scattered electrons here generally have an energy in the range of 50 eV up to the energy of the electrons of the primary electron beam at the object. The secondary electrons furthermore generally have an energy of less than 50 eV. The secondary electrons and/or back-scattered electrons are detected using at least one detector. The detector is arranged for example inside the objective lens or in a region between the objective lens and the beam generator. When the secondary electrons and/or the back-scattered electrons impinge on the detector, a detector signal is generated which is used to generate an image.
Imaging of an object using an electron beam device can be achieved with high spatial resolution. The high spatial resolution is obtained owing to the primary electron beam having a very low diameter in the plane of the object. Furthermore, the spatial resolution is the better, the closer the object is arranged to the objective lens of the electron beam device. Furthermore, the spatial resolution is the better, the greater the electrons of the primary electron beam in the electron beam device are initially accelerated and decelerated to a desired energy (examination energy) at the end in the objective lens or in the region between the objective lens and the object. The electrons of the primary electron beam are accelerated, for example, with an acceleration voltage of 2 kV to 30 kV and guided through an electron column of the electron beam device. The electrons of the primary electron beam are decelerated to the desired energy only in the region between the objective lens and the object, such that the electrons of the primary electron beam have, for example, an energy in the range of 10 eV to 30 keV, with which they impinge on the object.
There are objects, which, owing to their structure, can be examined in an electron beam device in a meaningful manner only if the electrons of the primary electron beam impinging on said objects have only a low energy, for example an energy of less than 100 eV. Electrons having such a low energy ensure that said particular objects are not destroyed when being irradiated by electrons. Electrons with such low energies are furthermore particularly suitable for generating an image of an object to be examined having a high surface sensitivity (that is to say with particularly good information content as regards the surface of the object). Moreover, electrons with such low energies are suitable for positioning measurements devices with pointy ends (what is referred to as a prober) at an object, in which electrical measurements are carried out using said prober. Electrons with higher energies than the above-mentioned low energies can possibly falsify such electrical measurements. Therefore, the use of electrons of the primary electron beam, which have such a low energy, are useful for investigating said objects.
However, it has been found that secondary electrons, which owing to the impingement of electrons of the primary electron beam are emitted by the object with an energy of less than 100 eV, are deflected into the electron column due to the acceleration voltage used for the primary electron beam and the field pattern or due to an existing magnetic field and/or electric fields for the electrons of the primary electron beam. Accordingly, a large portion of said secondary electrons can be used for imaging only if they are detected with a detector that is arranged in the electron column.
Such a detector is known from the prior art. Said known detector is constructed to be annular and has a relatively large opening. The optical axis of the electron beam device extends through the opening. The opening is necessary so as not to influence the primary electron beam in the beam path of the electron beam device and to avoid possible contamination.
Reference is made by way of example to DE 198 284 76 A1 and DE 10 2009 028 013 A1 as regards the prior art, which are incorporated herein by reference.
However, it has been found in examinations using electrons of the primary electron beam which have a low energy (for example less than 100 eV) that, owing to the field pattern already mentioned above, the paths of most secondary electrons travel very closely to the optical axis of the electron beam device, with the energies of the secondary electrons and of the electrons of the primary electron beam deviating only slightly from one another. Since the secondary electrons therefore have almost the same energy as the electrons of the primary electron beam, the secondary electrons travel on similar paths through the electron beam device as the electrons of the primary electron beam. Secondary electrons, which emanate from places of the object which, when scanning the object to be examined, are somewhat remote from the optical axis of the electron beam device, also travel in the electron beam device and in particular in the plane of the detector outside the optical axis. They are detected at least partially by the detector in the electron beam device. However, a large portion of the secondary electrons which emanate from places of the object close to the optical axis pass through the opening of the detector without being detected by the detector. Secondary electrons which emanate from places that are remote from the optical axis, by contrast, are incident on the detector. An image of the object to be examined is obtained, wherein the image practically has a detection hole in the middle, since no electrons are detected at this place and thus no image information is available.
Another problem is furthermore known from the prior art. It has been shown that, in the case of relatively small working distances of approximately 1 mm to 2 mm, the mentioned problem is very pronounced. An examination using electrons of the primary electron beam having an energy of less than 1 keV (for example 100 eV) is not possible or possible only with great difficulty in the case of short working distances using the known electron beam devices.
Moreover, it has been shown that an examination of an object using electrons of the primary electron beam having an energy of less than 100 eV is also dependent on the beam generator used. Not every beam generator allows such an examination. For example it is known to use a Schottky emitter for generating the primary electron beam. In a Schottky emitter, electrons are extracted from the Schottky emitter due to an extractor voltage of an extractor electrode. The extractor voltage is typically between 3 kV and 5 kV. Considerations have concluded that the acceleration voltage with which the primary electron beam is guided through the electron column of the electron beam device should be greater than the extractor voltage. However, the higher the selected acceleration voltage, the fewer secondary electrons impinge on the detector if the electrons of the primary electron beam have an energy of less than 100 eV when impinging on the object. The higher the acceleration voltage, the smaller the relative energy difference between the electrons of the primary electron beam and the secondary electrons. It follows that the secondary electrons basically travel along the same optical path as the electrons of the primary electron beam and therefore pass through the opening of the detector. The above-mentioned problem of the detection hole is very pronounced in the case of high acceleration voltages. Furthermore, the maximum working distance becomes increasingly small. The result in principle is that an image of the object using electrons of the primary electron beam having an energy of less than 100 eV is almost not possible at an acceleration voltage of greater than 4 kV. However, since the accelerator voltage is supposed to be greater than the extractor voltage of an emitter, this can lead to a problem, because beam generators are known which require an extractor voltage of greater than 4 kV. In that case, however, the accelerator voltage is also greater than 4 kV. As a result, examination of an object using electrons of the primary electron beam having an energy of less than 100 eV is almost impossible.
Accordingly, it would be desirable to specify a particle beam device and a method for operating a particle beam device, with which an object can be examined using particles having a low energy such that in particular good imaging of the object is achieved.